When I was young enough to have kids, I was mentally too much of a mess to have them--and I knew it. I'd probably be a great parent now, and I've got the money and stability to give a kid a decent life, but I'm just too fucking old to do it.
I've also never had any desire to be a parent. I wasn't against it, and I think that if I'd ever been stuck with a kid I would have at least tried, in my own stumbling way, to do right by them, because kids deserve that. I'm not one of those childfree people who hates kids; I'm generally inclined to like them, and want the best for them. But I never felt so much as a twinge of desire to get pregnant; never felt much of anything at the sight of babies; never dreamed of what life would be like as somebody's mom.
So the years went past, and having kids never happened, and I feel no regret about that. I look at my siblings, both of whom had kids late and now have young children, and they're really happy they did--far happier than they expected to be. I can understand that yes, they are privy to a whole set of feelings and an entire world of adult experience that I'm not, but I still don't feel like I'm missing out, or that I missed my chance, or that I should have done any different. So now I get to be the Crazy Old Auntie who paints her house in weird colors and has all the kitties, and we're all good with that.